东昆仑加鲁河中三叠世含石榴石二云母花岗岩的成因及地质意义

Petrogenesis and Geological Implications of the Middle Triassic Garnet-Bearing Two-Mica Granite from Jialuhe Region, East Kunlun

东昆仑造山带广泛发育晚古生代-早中生代富含暗色微粒包体的花岗闪长岩类,被认为是研究壳幔岩浆混合作用的天然实验室,然而同时代的过铝质-强过铝质型花岗岩在该地区鲜有报道.在东昆仑东段加鲁河地区新识别出一套含石榴石二云母花岗岩,为了探究其成因归属及地质意义,我们对其开展了详细的矿物学、岩石学、锆石U-Pb年代学、岩石地球化学及锆石Lu-Hf同位素研究.该套岩石呈多条近平行脉状侵入花岗闪长岩中,主要组成矿物包括石英、斜长石、钾长石、黑云母和白云母,副矿物中可见少量石榴石.LA-ICP-MS锆石U-Pb定年获得含石榴石二云母花岗岩成岩年龄为242.0±1.4 Ma,属于中三叠世岩浆活动产物.化学成分上,该岩石具有高SiO2(74.49%~75.24%)、高K2O(4.21%~4.33%)、低P2O5(0.02%~0.03%)含量和过铝质-强过铝质(A/CNK值为1.08~1.12)特征,表现出较高的分异程度.其U、Th、Pb、Rb等元素相对富集,Nb、Ta、P、Ti等元素相对亏损,其中P、Ti强烈亏损,这可能与磷灰石及钛磁铁矿的早期结晶分异有关.稀土元素总量较低为(61.55×10-6~119.05×10-6),有弱负Eu异常(δEu=0.51~0.65).锆石εHf(t)值变化范围为-8.19^-2.78(均值为-5.61),二阶段模式年龄(TDM2)为1.3~1.6 Ga,全岩Nb/Ta比值8.22~9.67(接近地壳比值10.91),这与区域内近同期的黑云母二长花岗岩特征相似,均指示该岩石可能源于下地壳的重熔.岩石中广泛发育的富云包体暗示岩浆上升途中捕获围岩物质,岩浆存在明显同化混染作用.综上,本文认为东昆仑加鲁河地区的含石榴石二云母花岗岩脉为一套过铝质-强过铝质的高分异I型花岗岩,是下地壳部分熔融产生的长英质岩浆经历长期结晶分异作用并在上升途中受到围岩同化混染作用之后的产物.

Late Paleozoic to Early Mesozoic I-type granodiorites enriched in mafic microgranular enclaves are widespread in Eastern Kunlun orogen,which therefore has been considered as a natural laboratory for studying magma mixing between mantleand crust-derived melts.In contrast,coeval peraluminum to strong peraluminous granitoids are rarely reported in this region.Recently,we identified a set of garnet-bearing two-mica granites in the Jialuhe region at eastern part of the East Kunlun Orogen.Hereby we present a synthetical mineralogy,petrology,zircon U-Pb geochronology,and lithogeochemistry and Lu-Hf isotope study on it,aiming to constrain its petrogenesis and tectonic implications.The rock outcropped as multiple sub-parallel dykes in the field,intruding into granodiorite.It mainly consists of quartz,plagioclase,k-feldspar,biotite and muscovite,with minor garnet as accessory minerals.LA-ICP-MS zircon U-Pb isotopic dating yields an age of 242.0±1.4 Ma,indicating that the garnet-bearing two-mica granites were formed in the Middle Triassic.Chemical analyses show that the rocks have characteristics of high SiO2(74.49%~75.24%),K2O(4.21%~4.33%),but low P2O5(0.02%~0.03%)contents,belonging to peraluminous to strong peraluminous series(A/CNK=1.08~1.12),showing a high degree of differentiation.They are relatively enriched in elements such as U,Th,Pb,Rb,but depleted in Nb,Ta and other related elements.Note that,P and Ti are strongly depleted,which may indicate early crystallization of apatite and titanomagnetite.The rocks also have a low total amounts of rare earth elements(61.55×10-6~119.05×10-6),and show a weak negative Eu anomaly(δEu=0.51~0.65).Their zirconεHf(t)values ranged from-8.19 to-2.78(mean of-5.61),with a two-stage model ages(TDM2)of 1.3 Ga to 1.6 Ga Ma and whole-rock Nb/Ta ratios are 8.22~9.67(close to continental crust ratio of 10.91).These features are similar to those of nearly coeval biotite monzogranite in the region,indicating that the rocks may be originated from remelting of lower crust.The biotite-rich enclaves are widely developed in the rocks,suggesting that the wall rock materials are captured during magma ascent,with obvious assimilation and contamination.Thus,we propose that the garnet-bearing two-mica granitic dykes belong to high-fractionated and peraluminous to strongly peraluminous I-type granites series,which are products of felsic magma produced by partial melting of lower crust undergoing longterm fractionation and being subjected to surrounding rocks assimilation during its ascent.